Accommodating an aging world will pose significant economic and social challenges and will ultimately call for both paradigm-shift in our understanding, and biomedical interventions into the aging process. Extensive data demonstrate that when the systemic niches of organ stem cells are biochemically rejuvenated, stem cells endogenous to multiple old organs engage in productive tissue regeneration. Importantly, the old circulatory milieu rapidly and significantly inhibits the regenerative performance of endogenous stem cells in young organs. This proposal uses what is known about the role of the circulatory milieu in the rejuvenation of aged tissue repair, with the goal o determining the key molecular mechanisms that are responsible for the phenomena of heterochronic parabiosis. Until recently, it was technologically impossible to specifically label and interrogate the proteome of one animal in the setting of heterochronic parabiosis; however the development of cell-selective metabolic labeling of proteins with non-canonical amino acids via expression of mutant aminoacyl-tRNA synthetases in mammalian cells by the co-PI (David Tirrell) enabled this paradigm shifting approach. This high-risk high-reward project becomes feasible due to the united efforts, areas of expertise and experimental models of the Conboy and Tirrell research groups. The Conboy team has proficiency in studies of heterochronic parabiosis and characterization of the effects of defined factors on tissue regenerative capacity, whereas the Tirrell laboratory has pioneered and time-resolved and cell-selective proteomics. The paradigm changing outcomes of these studies are many fold: (1) revealing the youthful pro-regenerative and aged inhibitory proteomes of blood serum, (2) uncovering the mechanisms by which the circulation influences the regenerative performance of organ stem cells in muscle and brain/hippocampus; (3) generating a comprehensive data-base that is required for understanding the genetics of aging and importantly, (4) identifying novel ways to rejuvenate multiple organs and extend healthy life span via systemic administration of defined molecules that emulate the physiologically young blood circulation.